The present invention relates to a system and a method for optimizing the generation of electrical power into electrical power mains. The present invention also concerns an electronic optimization controller for optimizing power generation into the mains by a synchronizer.
A synchronizer is a device which enables the controlled generation or injection of electrical power from an asynchronous generator, rotating at variable speeds, into a main network which has a predetermined fixed voltage and frequency.
In essence a synchronizer is a modification of a four-quadrants current link frequency converter. It is known that current link frequency converters which drive motors with large inertia loads (such as centrifuges) at variable speed, are used to brake these motors by a passive, uncontrolled regenerative braking action, by loading the motor and feeding the resulting generated power to the net until the motor is braked. The modification enables the synchronizer to be used for new applications and is installed between the main electric network and the asynchronous generator. The synchronizer enables recovery of energy from systems out of which energy could not be readily or economically recovered before, and also greatly improves the performance of conventional energy recovery systems.
In industrial plants, water works and the like, situations may arise where fluids with excessive energy, i.e., excessive pressures and velocities, are available. This surplus of energy may be converted into electrical energy, and be used. If this excessive energy is not collected and used, it will usually be throttled by valves, by-passed or dissipated by other means, and be wasted.
Typical examples where such excessive available energy could be advantageously utilized are:
Reinjection, in winter time, of surplus water back into the ground to replenish the underground water tables; branching off oil or water lines from high pressure main lines into low pressure zones of local users areas; bleeding off product or crude oil from high pressure point in a line to storage; reverse osmosis rejected brine; windmills, and exploitation of the head of water sources in hilly terrain.
The universal and most economic way to recover such energies is to drive a generator by a fluid turbine and to inject the generated electrical energy into the main elecric network. One of the most economical and simplest generators is an asynchronous motor, driven at over-synchronous speeds, and this invention is mainly directed to this type of generator. The turbine may be of the Pelton or Francis wheel type, however, the most economic and simplest turbine is a fixed vanes centrifugal turbine, which is, in essence, a reversible pump running as a turbine. This centrifugal turbine is especially convenient when using dual purpose units, i.e., units that operate both as motor driven pumps for part of the time, adn then as turbine driven generators for the rest of the time. When fluid pressure is reversed, the pump becomes a turbine and the motor becomes an asynchronous generator.
Since the main network is a "rigid" system, i.e., a system having a fixed frequency and voltage, the output of the asynchronous generator is to be regulated in such a way that its rotational speed values are kept within close tolerances in the vincinity of its synchronous speed, corresponding to the main network and slightly above it.
If these tolerances are not kept, two things may happen:
1. The asynchronous generator will "over-run" the mains, and will break away out of synchronization; and PA0 2. The asynchronous generator will "drag" or "fall back" realtive to the net, and will then be driven by the net operating as a motor and consume energy, instead of generating energy and feeding it into the net. PA0 (a) synchronizing the frequencies of said synchronizer to the frequency of the free-running generator; PA0 (b) changing the frequency value of said generator to a lower or high frequency value of a predetermined period of time; PA0 (c) determining the direction in which the rated value of the generator's frequency has to be changed for achieving maximum available power; PA0 (d) repeating steps (b) and (c) until said maximum available power is attained, and PA0 (e) changing the generator's frequency and thereby the generator's speed to generate power to the mains at said maximum available power. PA0 sample and memory means for continuously monitoring and storing signals representative of the actual instantaneous generated output power; PA0 a comparator for comparing an instant output power signal with a previous output power signal and providing a signal indicative of the direction and magnitude of a power output change; PA0 a memory connected to said comparator for storing the signal indicative of the direction of the power output change and the direction of the frequency change of said generator; PA0 a logic circuit fed by said memory for determining the direction of the frequency change to be induced in the generator resulting from the last change in the generator's power output and in the last change in the generator's frequency; PA0 circuit means connected to said memory and responsive to a decrease of power output of the generator, which follows a previous increase of power, by emitting an output signal activating a delay circuit for keeping the generator at a constant speed for a predetermined period of time; PA0 power change magnitude detection circuit, connected for receiving signals from said comparator and adapted to activate said logic circuit whenever the magnitude of a signal representative of the power change exceed a predetermined value, and a trigger circuit, connected to the output of said logic circuit, for triggering the synchronizer's inverter for changing the generator's speed in a direction determined by said logic circuit.
Due tot he above reasons, in this kind of an installation two main aspects are to be considered: (a) how to enable continuous running of the system under varying turbine conditions without breaking out of synchronization; and (b) how to maintain good system efficiency under varying turbine conditions.
Regarding thd first aspect, since the generator is directly connected to the main grid or network, this main network is imposing a constant speed on the turbine-generator system, however, contrary to this, the fluid conditions may very over a wide range. In order for the system not to "break-away", from, or to "drag behind" the main grid, in spite, and irrespective of varying hydraulic conditions of the fluid which drives the turbine, one of two solutions is usually used:
1. When using uncontrollable turbines, such as reversed pumps, a control system is necessary between the source of the fluid and the inlet of the turbine in order to keep the turbine conditions within the required tolerances. This sytem is adapted to throttle or by-pass (and therefore waste) excessive fluid flows and pressures, in case fluid conditions are too "high", in order to maintain the imposed fixed speed or, to disconnect the generator from the mains in case fluid conditions are too "low" to maintain this fixed, imposed speed, thereby by-passing and wasting the fluid energy which is available. Such control systems are inefficient, as they must dissipate or relieve fluid pressures and flows that could otherwise be used for energy generation.
It also has to be taken into consideration that at fixed, imposed speed, and reduced fluid pressures and flows, the efficiency of fixed vanes centrifugal turbines and reversed pump falls extremely rapidly, until it falls below zero (consumption of energy) and the system has to be disconnected from the mains.
In some cases, like wind turbines, it is altogether impractical to install a regulation control system on the driving fluid. As a result, alternating current cannot be effectively used and hence, direct current systems are instead installed, or alternatively, a complex vane angle changing control mechanism is to be used.
2. When using controllable turbines such as a Pelton Wheel turbine with adjustable nozzles, or a Francis turbine with adjustable vanes, the turbine can be controlled over a certain range of fluid conditions, however, again, the system still has to be disconnected when fluid conditions are not sufficient to maintain the fixed speed and fluid has to be by-passed in case fluid conditions are too "high". These kind of turbines are costly to maintain due to sealing problems at the adjustable vane control rods, which rods can only operate reliably with clean fluids.